My rule of thumb is that anything title "...for Hackers" is crap.This link was not an exception.

> Since the relative error is invariant under scaling (x↦αx)(x \mapsto \alpha x)(x↦αx), we will mostly be interested in relative error.

No, No, No, No. This is very much dependent on what you are measuring/modelling, what is the tolerance you are willing to accept, the consequences and so on.

A 1% of error in the change the cashier gave you back maybe is not a big deal. That same error in the national budget is equivalent to 2X the NASA's budget.